I guess I had a million ideas during the last decade of how Reno and Rude get from friendship to relationship. One, rather simple one, of these ideas is that Reno is actually running around with feelings for quite some time, unable to find the courage or to express what he actually thinks. He might have tried to talk to Rude when he was drunk but Rude didn't take it serious at all, not even thinking about the possibility that he's telling the truth. Maybe because Reno's usually one big idiot in general when he's drunk.